The present invention relates to methods and apparatus for implementing arbitrary concatenation of frames conforming to a synchronous optical network (SONET) standard.
SONET has been adopted as a standard protocol for fiber optic transmission, whereby data is transmitted as a series of time slots or frames. Higher data rate SONET transmissions multiplex more frames than lower rate transmissions. The lowest SONET data rate transmission, however, transmits at a base rate of 51.84 Mbit/second. Frames associated with this fundamental SONET transmission are referred to as synchronous transport signal level one (STS-1) in the electrical domain, and the corresponding optical signal is referred to as an OC-1.
Higher data rate SONET frames are formed of integer multiples of STS-1s, and have designations STS-N, OC-N for the corresponding optical signals, where N is 3, 12, etc. Each OC level has a corresponding data transfer rate that is a multiple of the base rate. OC-3, for example, runs at three times the base rate.
As seen in FIG. 1, one such high speed SONET signal, OC-48, includes 48 STS-1 frames, when converted to corresponding electrical signals. Each STS-1 frame is transmitted during a respective time slot, and comprises two components: transport overhead and payload. Transport overhead is provided in 9 rows of three bytes each (27 bytes total), and carries administrative information used by network entities to manage the transfer of the frame through the network. The payload, referred to as the Synchronous Payload Envelope (SPE), is provided in 9 rows of 87 bytes each (783 bytes total) and comprises the major portion of an STS-1. The SPE carries payload and STS Path Overhead (STS POH) bytes, and may begin at any byte location within the payload envelope, as indicated by a pointer value in the overhead.
Certain broadband transmission protocols (e.g., ATM and ISDN), however, include relatively large payloads which do not fit within a single STS-1. Thus, in order for these protocols to be transmitted over SONET, a plurality of STS-1 s are concatenated together. Such concatenated STS-1 are referred to as STS-Nc, and are multiplexed, switched and transported as a single unit. The SPE of an STS-Nc includes N×783 bytes, which may be considered as an N×87 column×9 row structure. Only one set of STS POH is used in the STS-Nc, with the pointer always appearing in the transport overhead of the first of the N STS-1 s that make up the STS-Nc.
The SONET standard, however, requires that the STS-1s that make up an STS-Nc occupy specific time slots. For example, FIG. 2 illustrates 48 time slots occupied by 16 OC-3cs transmitted within an OC-48 frame. In particular, as seen in FIG. 2, OC-3c #1 must occupy a “row” of time slots 1, 17 and 33, OC-3c #2 must occupy time slots 2, 18, and 34, and OC-3c #16, must occupy time slots 16, 32, and 48. In order to add a new OC-3c, one entire row shown in FIG. 2, must be removed or reallocated.
FIG. 3 illustrates specific time slots occupied by four OC-12cs within an OC-48 frame. Specifically, OC-12c #1 must occupy time slots 1–4, 17–20, and 33–36, OC12-c #2 must occupy time slots 5–8, 21–24, and 37–40, and OC-12c #4 must occupy time slots 13–16, 29–32 and 45–48. Likewise, in order to add a new OC-12c, an entire row shown in FIG. 3 must be removed or reallocated.
If time slots 1, 2 and 3 are dropped in the OC-48 frame shown in FIG. 3, and populated with data for an OC-3c, however, they could not be switched by current SONET equipment because they are not transmitted in a sequence conforming to the concatenation protocol described above. Rather, a conventional SONET network element would need to be reconfigured to thereby rearrange the remaining time slots so that a new OC-48 frame is created which does conform to the standard concatenation sequence. If reconfiguration is not performed, the empty time slots cause bandwidth fragmentation.
Reconfiguring SONET network elements, however, requires substantial down time causing disruption in the flow of data through a network. Thus, there is a need for a network element, such as a switch, which can arbitrarily concatenate time slots associated with an OC frame which are not provided in a given “row” or sequence required by SONET.